The two solutions work independently, but complement each other.
Basically, when I'm sitting at my computer listening to music through MC - then the MC Last.fm plugin does my scrobbling for me.
Then, when I'm up in my living room, I have MC serve my Soundbridge through UPnP. The sbPopper (running on same computer as MC) detects when the Soundbridge plays a song and scrobbles it for me. Works well.